An all-metal, bakeable, ultra-high vacuum reflection high-energy electron diffraction (UHV RHEED) system has been developed for insitu studies of films during deposition on surfaces by processes such as r.f. sputtering, where pressures in the region of the specimen may be as high as 3-4 Pa. The design separates the 50 kV gun vacuum from that of the specimen chamber with a single differential pumping aperture, which also serves as the beam-defining aperture. The electron optics enable a focussed spot of about 50 mu m to be produced on the detector at a distance of 50 cm from the end of the magnetic lens housing, with adequate current for RHEED studies. The RHEED system has been installed in an UHV r.f. magnetron sputter-deposition chamber and has been successfully applied to (i) monitor, in the presence of the sputter discharge, the surface of a chemically cleaned n-type Si(100) wafer and (ii) study the subsequent growth of Pt films on the silicon surface under different conditions of sputtering pressure and r.f. power.